1. Field of the Invention
The present invention relates to a flow through pressure transducer designed for harsh environments, such as for example the environment found in an oil well cement slurry mixing process.
2. Description of the Related Art
Pressure transducers that screw into a tap on the outside of process pipe are available in the industry. These transducers work for only a limited time and then fail for one of two reasons. The first reason that prior art transducers fail is that if the end of the transducer sensor is placed into or very close to the flow stream, the cement will wear the diaphragm on the end of the sensor and thereby causing it to fail. The other reason that prior art transducers fail is that if the sensor is placed farther away from the flow stream, the sensor can become fouled with set cement and therefore render it useless. The biggest problem in the past when using a pressure transducer in an oil well slurry mixing process has been the survival of the gauge in the cement slurry environment.
Also, most transducers are sealed units where repair is difficult, if not impossible.
The present invention addresses these problems with a new flow through transducer design. The invention employs a thin cylindrical sensor sleeve to sense the pressure. The thin metal sleeve is protected from potential fluid wear and corrosion by a molded elastomeric sleeve on its internal diameter. The elastomeric protective sleeve is molded into the internal diameter of the sensor sleeve such that no fluid reaches the sensor sleeve. Pressure is transmitted to the sensor sleeve through the elastomeric sleeve. The elastomeric sleeve protects the cylindrical sensor sleeve from both wear and corrosion from fluids passing through the transducer.
The sensing element is a thin metal sleeve that is unrestrained in the axial direction, thereby preventing the sensing element from being subjected to axial pressure induced stresses. No structural loads are transmitted to the sleeve. The sensor sleeve is mounted in such a way that the sensor internal pressure only loads the sleeve in a hoop stress direction and subjects the sensor sleeve only to hoop stresses.
Pressure exerted on the inside of the sensing element causes hoop stress on the sensing element. The strain gauge senses this hoop stress. Thus, the output of the strain gauge is proportional to the amount of pressure exerted on the inside of the sensing element by the fluid flowing through it.
Two dual strain gauges are mounted on the outside of the sensor sleeve in spaced apart relationship. Each dual strain gauge has a gauge for measuring hoop stress and a second strain gauge mounted at a 90 degree angle to the first strain gauge for providing temperature compensation. A bridge output amplifier is used to provide a stable power supply input to the gauges, to amplify the signal, to allow zeroing the output and to adjust the output span.
The present invention was designed for mounting in a 3″ pipe with a working pressure of 150 psi. However other sizes, larger or smaller, could be easily made based on this design, and the sleeve thickness could easily be increased to measure higher pressures.
The body of the transducer forms a spool surrounding the sensor sleeve. The spool is held in place by transducer outer flanges that secure to the spool via threaded fasteners. In case the sensor sleeve failed, the body of the transducer which surrounds the sensor sleeve would catch any escaping fluid and prevent fluid from discharging outside the sensor. The spool assembly also prevents exterior damage to the thin sensor sleeve.
The present invention improves over the prior art in that it cannot become plugged with cement or other material and its sensor sleeve is protected from fluid erosion and corrosion by its molded elastomeric inner sleeve.                Further, the present transducer can be disassembled and repaired. This allows the sensor sleeve assembly of the present invention to be replaced with different pressure rated sleeves and with sleeves containing elastomers with different chemical compatibilities.        